Shielded spark plug cable terminal



J. J, ROSE y April 2s, 1942.

PLUG l CAELE TERMINAL SPARK SHELDED Filed Oct. l5,

' /A/l/c/vra@ L/OH/V d. o sf ZL fr fToQA/pr Patented Apr. 28, 1942UNITED STATE S PATENT OFFICE (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. '157) IZClallns.

'Ihe invention described herein may be manufactured and used by or forGovernment for governmental purposes. without the payment to me of anyroyalty thereon'.

This invention relates to improvements in the cable terminals forshielded sparky plugs whereby the life of the high tension cablesconducting the high voltage sparking current to"`the plugs is greatlyextended.

Shielded spark plugs as now generally used on aircraft engines for-ignition purposes include in addition to the plug proper, a groundedtubular metal shield into which the upper parts of the spark plug coreand central electrode project, the inner surface of the shield beinglined with a suitable insulating material. The spark plug cable isformed oi' a central conductor surrounded by rubber insulation which inturn `is covered by a braided fabric sleeve coated with lacquer torender it waterproof and to protect the rubber insulation againstoxidation. The terminal end of the cable is pushed into an annularsleeve or thimble 1v1-ade of Bakelite or ceramic insulating materialwhich is provided on its end with a suitable contact to which the cableconductor is connected. The lcable terminal ywith the assembledinsulating sleeve is then pushed into the-tubular shield of the plug,the contact on the sleeve engaging a contact secured to the upper end ofthe central spark plug electrode. The spark plug cable is then pulledlthrough a curved tubular metal elbow forming part of the shielded plugand which is secured by a. union nut to the threaded upper portion ofthe shielding tube, the

. Joint being sealed by a resilient rubber sealing washer whichencompasses the cable; The above noted elbow at its outer end serves asa connection for the usual braided metal conduit or harness throughwhich the ignition cable is passed.

A great deal of difficulty has been experienced with shielded ignitionapparatus of the character described which results from severaldifferent causes, among which are the high temperatures existingadjacent the plugs due to heat transmission therethrough; hightemperatures in the shield elbows, due to the latter being in thevicinity of the engine exhaust conduits; moisturercol- Y lecting alongthe cable, and especialLv at the plug terminal; necking of'theinsulation on the cable by the sealing washer, causing damage to thelacquer protecting the fabric covering of the cable; and damage to thelacquer coating in pulling the cable through the shield elbow.

In accordance with the present invention many of the above troubles areminimized thus greatly extending the service life of the ignitioncables, and the improvements in accordance with the invention may beattained by simple means without changing any of the standard ignitionunits now employed in the art.

lite" or ceramic sleeve, or thimble, generally em-` ployed on theterminal end of the cable is retained and the cable itself is preferablychosen of a smaller outside diameter than would ordinarily be employedwith the respective terminal sleeve or thimble. A thin walled rubbertube is slipped over the cable forich a length that the tube will extendat least to or beyond the outer end of the shield elbow when assembled.Aiiat rubber washer is placed over the cable conductor at the terminalend thereof so as to abut the end of the rubber tube and braided fabriccovering on the cable. The end of the cable is then forced into theconventional terminal thimble with the rubber washer tightly seated onthe bottom thereof thus sealing the cut end of the braided covering ofthe ignition cable against moisture. The protective rubber sleeve servesto materially protect theignition cable proper from damage from hightemperatures both.with in the plug shield and in the shield elbow. andfurther distributes the sealing washer pressure at the junction of theplug shield and the shield elbow, preventing necking and fracture of thelacquer coating on the cable. The rubber sleeve also protects thelacquer coating on the cable against damage while being pulled throughthe shield elbow and is further effective to protect the cable againstthe effects of high temperatures in the vicinity of the shield elbow dueto heat from the engine exhaust conduits. The protection of the lacquercoating on the cable at those points where the same is liable to bemechanically damaged is oi great importance because damage or abrasionof the lacquer coating will allow moisture to enter the braid whichpermits a corona discharge, causing the production of ozone whichrapidly oxidizes the rubber insulation of the cable causing the same toswell and crumble and necessitating frequent renewal of the igniil tioncables.

'Ihe principal object of the invention is therefore the provision of aspark plug terminal for ignition cables employed in conjunction withshielded plugs which provides a greater protection to the cableinsulation in those portions adjacent and within the plug shieldingelements subject to high temperatures. and insures adequate protectionof the insulating cover of the cable from moisture and mechanical damageor abrasion.

A further object of the invention is the proyision'of a terminal forhigh tension ignition cable employed with shielded spark plugs;comprising a rigid thimble of insulating material having a contactthereon adapted to be electrically connected to the cable conductor, awasher of rubber or like material adapted to encompass the projectingportion of the cable conductor and to abut the end face of the cableinsulation in sealing relation thereto, and a sleeve of rubber or likematerial surrounding the cable for a length at least sufficient toprevent contact of the cable with the shielding elements of the sparkplug and the walls of the thimble and at its lower-end engaging saidsealing washer in sealing relation.

Other objects of the invention will appear by reference to the detaileddescription hereinafter given and to the appended drawing in which:

Fig. 1 is a side elevation partly in section illustraurig the novelcable terminal assembled in a shielded spark plug, and;

Fig. 2 is a sectional view to an enlarged scale illustrating the detailsof the terminal of Fig. 1;`

and

Fig. 3 is a view similar to Fig. 1, illustrating a modified form of theinvention.

Referring now to Figs. 1 and 2, the reference numeral I, generallyindicates a conventional type of shielded spark plug employed inaircraft engines and having a metal body 2, insulating core y3 and acentral high tension electrode I provided at its upper end with acontact button 5. The plug is provided with the usual grounded metalshield 6 screwed into the upper end of the spark plug body 2 and linedwith an insulating material such as fBakelite or a ceramic composition,as indicated by the reference numeral 1. The upper end of the tubularshield 6 is threaded as at 8 to receive a coupling 9 which serves toconnect a tubular shield elbow IIB to the shield 6, the outer end of theelbow being suitably threaded to receive the coupling member of awell'known type of braided metal conduit II which serves to electricallyscreen the cable between the distributor and the spark plug.

The high tension cable is generally indicated by the reference numeralI4 and comprises an electrical conductori5 covered by a thick layer ofrubber insulation I6, which in turn is protected by an external layer ofbraided fabric Il coated with lacquer which protects the braid frommoisture, and if undamaged, greatly minimizes the possibility of coronadischarge which causes rapid deterioration of the rubber insulation.

The spark plug terminal on the cable comprises an annular insulatingsleeve or ferrule I8 made of Bakelite or of ceramic materialsubstantially closed at its lower end by a moulded in metal washer ordisc I3 to which the cable conductor yi5 is soldered or otherwiseelectrically connected on assembly. The disc I is provided with acontact 2U in the form of a spring which is adapted to engage thecontact on the central spark plug electrode 4. l The ierrule or sleeveI8 with the closure I9 forms av thimble which encloses the end of thecable and is a sliding nt in the tubular shield l. The cable Il insteadof directly contacting the thimble vI8 is separated therefrom along thebottom face of the cable by a sealing washer 2l made of rubber orsimilar material which seats on the bottom I9 of the thimble land abutsthe insulation I6 and cut end of the braided fabric covering I1 of thecable in water tight relation, preventing the entrance of moisture onthe frayed end of the covering I1 and tightly encompassing the conductorI5 of the cable. The cable is further insulated from the inner walls ofthe thimble by a tubular rubber sleeve 22 which extends along the cablefor a length suiilcient to pass through the shield elbow I0 into themetal conduit or shielding harness II. A rubber or other resilientpacking washer 23 encompasses the sleeve 22 and serves as a packing sealfor the coupling S.

In practice the cable I4 is chosen of a diameter slightly smaller thanwould otherwise be employed and the tubular sleeve 22 made of rubber orsimilar material is slipped over the end of the cable, the sleeve thenbeing inflated with air under pressure is slipped on the cable for thedesired length. The sleeve 22, cable covering I1 and insulation I6 ofthe cable Il is cutV transversely and the proper length of conductor isleft projecting for connection to the spring contact 2li. The rubberwasher I9 is then forced over the projecting portion of the conductorand seated against the transverse end faces of .the sleeve 22 and theprotective covering and insulation ofthe cable, and may be cemented tothe tubular sleeve 22 to form a bond therewith. After passing the cablethrough the elbow Il the insulating thimble I8 is forced over the end ofthe sleeve 22 until the washer 2l is tightly seated on the closuremember I9, the conductor I5 then being connected to the contact 2l andthe thimble I8 inserted in the plug shield l to completethe assembly.

The tubular sleeve 22 protects the lacquer coating on the cable frommechanical damage while being pulled through the shield elbow IU andfurther materially protects the cable covering from the effects of hightemperatures in the vicinity of the elbow I0 due to heat from the engineexhaust manifolds. The portion of the sleeve 22 within the thimble I8serves in conjunction with the washer 2l to protect the cable againstany entrance of moisture and further materially reduces the effects ofheat transmitted from the plug body and core from. damaging the lacquercoating on the cable exterior. The sleeve 22 further serves todistribute the pressure of the sealing washer 23 so that the cableproper is not necked beneath the washer and the lacquer coating at thispoint is not damaged as frequently occurs in prior constructions.

The modified form of the invention illustrated in Fig 3 is similar inall respects to the construction of Figs. I .fand 2 except that theferrule or thimble I8 is'shortened and a second tubular sleeve 24 isplaced over the sleeve 22 and terminates at its upper end in anintegrally moulded radially extending sealing washer 25 which serves thesame function as the sealing washer 23 of Figs. l and 2, but moreeffectively distributes the sealing pressure to eliminate necking ofthecable and to prevent moisture from passing along the cable to theterminal connection thereof.

It is to be 'understood that while the invention has been illustrated asapplied to spark plugs employing shielding elbows, the terminalconstruction is equally applicable to those plugs in which the ignitionharness is coupled directly to the plug shield and in such cases thetubular sleeve is made of such a length that the cable is well protectedadjacent the coupling member and 4constri'ictio'n :isf -standpointz Itisfinther-to i vfor 2a considerable :distance-beyond' from lieat--.` Ytransmittedfrom;` thefexhaustaswell as froni'rr mechanical abrasion due"to' bending *of the cable to-makefc'oupling connectionss': It is-furthernoted; that 'fthe Masher"- 21 may be integrally.foi-meri*withfthesleevefinv though the separate?.

preferableffrom' fassembly- 'L1 f "insulation, protective covering andsaid sleeve, a

- `tseunderstood mit the temi `rubber as? referringf1toftheItubular.protecting equal-toer greater than the length ofthe shielding. `elementsof an associated-spark plug, said .sleeve being-slippedronover saidcable to surround the. terminal fend thereof, a resilient sealing'washer -adapted tou.; tightlyv encom- .pass -the projecting; portlonhoftheycable conductor and. to'abut the end faces of the cable -1 rigidferrule `of insulating material having a subsleeve and sealing disc isintended to refer to' having properties somewhat similar to naturalrubber and generally referred to in the art as "synthetic rubber.

Though preferred forms of the invention have been illustrated anddescribed other forms and modications will become apparent to thoseskilled in the art as falling within the scope of the invention as denedby the appended claims.

I claim.

l. A moisture proof terminal construction for ignition cables of thecharacter used in conlunction with shielded'spark plugs having agrounded shielding sleeve projecting from the plug body; comprising anannular thimble of a rigid insulating material having a substantiallyclosed lower end provided with a contact adapted to engage the centralelectrode of the spark plug, said contact being adapted to beelectrically connected to the cable conductor, a resilient sealingwasher seated on the bottom of said thimble and adapted to tightiyencompass the conductor of said cable and to abut the end face of theinsulation and protective covering thereof in sealing relation, and atubular sleeve of rubber or like material surrounding said cable, theend face of said sleeve also engaging said washer in sealing 'relationand the said tubular shield tightly engaging the interior walls of saidthimble in sealing relation, said tubular sleeve extending along saidcable from the said thimble a distance sufficient to prevent contactbetween said cable and any of the shielding elements of the associatedspark plug.

2. A terminal construction for high tension cables of the characteremployed with shielded spark plugs having grounded tubular shieldingelements extending from the body of the plug; comprising a tubularrubber thimble adapted to encompass the exterior of the cable for alength equal to or greater than the length of the shielding elements ofthe associated spark plug, the said thimble abutting the end face of theinsulation and protective covering of the -cable in sealing relation andseallngly encompassing the projecting portion of the cable conductor, aninsulating ferrule made of a rigid insulating material having asubstantially closed end provided with a contact adapted to beelectrically connected to the cable conductor, said ferrule beingadapted to be forced onto said cable over the end portion of saidtubular rubber thimble and the contact thereon being adapted to engagethe central spark plug electrode when the ferrule and cable are insertedin assembled relation into the shielding elements of the plug.

3. A moisture proof terminal construction for high tension shieldedspark plug cables of the character in which the electrical conductor lssurrounded by insulation made of rubber or the like and covered by abraided fabric material coated with a water proofing material;comprising a tubular sleeve madeof a resilient water proof insulatingmaterial having a length bothvnatur'al rubber and the various compoundsstantially closed end provided withan electrical contact', said. ferrulebeing forced onto the end of said'fcable. overr. the tubularV sleeveuntil the sealing washer is compressed against the closed end of theferrule, and an electrical connection between said contact and theconductor in said cable.

4. The structure as claimed in claim 3, including a second tubularsleeve surrounding said rst named sleeve the lower end of which engagesthe upper end of the said ferrule, and a radially extending annularsealing washer integrally formed with said second sleeve and serving asa packing seal for the junction of the spark plug shield with othercable shielding elements.

5. A terminal construction for high-tension cables `of the characteremployed with spark plugs having grounded tubular shielding elementsextending from the body of the plug; comprising a tubular thimble ofrubber or like material adapted to encompass the exterior of the cablefor a length equal to or greater than the length of the shieldingelements of the associated spark plug, said thimble abutting the endface of the insulation and protective covering of the cable in sealingrelation and sealingly encompassing the projecting portion ofthe cableconductor, an insulating ferrule made of a rigid insulating materialhaving a substantially closed end provided with a contact adapted to beelectrically connected to the cable conductor, said ierrule beingadapted to be forced on to said cable over the end portion of saidthimble and a resilient tubular sleeve made of rubber or like materialsurrounding said thimble in sealing relation and abutting the upper endof said feriule, the upper end of said sleeve having integrally formedtherewith, an annular radially extending sealing element adapted toserve as a seal for the junction of the spark plug shield .with othercable shielding elements.

6. In a terminal construction for high tension ignition cable for use inconjunction with shielded spark plugs, said cable being of the characterin which the Velectrical conductor is surrounded by insulation of rubberor like material; comprising a rigid feri'ule of insulating materialhaving a substantially closed end provided with an electrical contact,said ferrule being forced over the end of the cable, an electricalconnection between the said contact and the cable conductor, a resilientcylindrical tubular sleeve made of rubber or like material encompassingthe cable in sealingrelation, one end of'said sleeve being placedinabutting and sealing relation with the top lof said ferrule and theupper end of said sleeve being provided with an annular integrallyformed radially extending disc or washer adapted to serve as a packingseal for the junction of the shielding `well of the spark plug withother cable shielding elements, the overall length of said tubularsleeve being substantially equal to or greater than the outside diameterof the sleeve so that radial pressure exerted on the sealing washer willbe distributed into the sleeve without necking the protective insulationof the cable.

of said thimble in sealing contact, said sleeve having formed thereonadjacent its upper end, a radially extending annular disc or washeradapted to serve as a packing seal for the shielding well of the sparkplug with other cable shielding elements, said sleeve having an overalllength materially greater than the thickness o! said washer portion soas to distribute the radial stresses on the washer into the body of thesleeve to thereby reduce necking oi the insulation on the cable.

JOHN J. ROSE.

